U.S. Pat. No. 3,679,000 discloses the use of various N-sulfohydrocarbon-substituted acrylamides for controlling the viscosity of aqueous media used in water-flooding operations for enhanced petroleum recovery. The polymers are described as containing 5-95%, by weight, of the N-sulfohydrocarbon-substituted acrylamide, preferably 50-95%, by weight, and even more preferably 70-95%, by weight. Comonomers such as acrylic monomers e.g. acrylic or methacrylic acid or salts, acrylamide, methacrylamide etc., are taught.
While the polymers discussed above and other commercially used polymers such as acrylamide-ammonium acrylate copolymers and polysaccharides have usually proven to be effective for the control of the viscosity of aqueous media, in many instances, the viscosity of the copolymers or material breaks down when the temperature of the well interior at the petroleum bearing deposit reaches above about 60.degree. C. i.e. preferably about 60.degree. C.-120.degree. C. or, alternatively, when prepared and/or used in the presence of oxygen, or both. Furthermore, the presence of divalent metal ions in the water already present in the wells or in that used to flood the wells during the enhanced oil recovery also tends to participate in the degradation of the copolymers and other materials which are added thereto. Such divalent metal ions are normally found in hard waters.
It therefore would solve a long-felt need if a method could be discovered for treating wells which exhibit temperatures above about 60.degree. C. i.e. those preferably ranging from about 60.degree. C. to about 120.degree. C., during secondary oil recovery, especially in the presence of mineral water containing divalent salts without the substantial accompanying break-down of polymer viscosity.